1. Field of the Invention
The present invention relates to an ink jet printing apparatus and ink jet printing method, more specifically to a structure for high quality printing and high-speed printing.
2. Description of the Prior Art
Recently, machines used in an office such as personal computers, word processors and the like are being widely used, and various printing apparatus for printing information processed by these machines are provided. High image quality and high-speed printing technologies for such apparatus are being rapidly developed.
(1) Image quality improving technology PA0 (2) High-speed printing technology
As an example of image quality improving technology, a so-called multi-scanning method is known.
When printing using a printing head having a plurality of printing elements such as ink ejection ports and the like, quality of a printed image depends very much on properties of the printing head itself. For example, minute differences generated in a production process of the printing head, such as a difference between shapes of respective ejection ports forming the printing elements of the printing head and deviation of a mounting position of an electro-thermal converter (ejection heater), individually affect an ejection amount of an ink and an ejection direction of the ink, resulting in degraded image quality as unevenness of density in a printed image.
A practical example thereof will be described with reference to FIG. 1 and FIG. 2. In FIG. 1A, a reference numeral 101 denotes the printing head, which is schematically shown and for simplicity, assumed as having eight ink ejection ports 102. A reference numeral 103 individually indicates an ink droplet ejected from the ejection port 102, which is assumed to be normally ejected in about the same ejection amount and in the same direction, as shown in the Figure. By such ejecting, a dot of approximately the same size is formed on a paper as shown in FIG. 1B, thereby obtaining a uniform image without the unevenness of density in the printed image as a whole (see FIG. 1C).
However, in practice, as described above, the individual ejection ports of the printing head often have deviations in ejection characteristics such as the ejection amount or the like. Therefore, when ejecting the ink without taking any measures, as shown in FIG. 2A, deviations are generated in sizes and directions of ink droplets ejected from the individual ejection ports. Then, dots formed on the paper differ in positions and sizes as shown in FIG. 2B. As a result of this, for example, unprinted white portions extending in a main scanning direction of the head may be periodically generated, in which portions dots are not formed to satisfy an area factor of 100%, or on the contrary, dots may be excessively overlapped, or a white stripe may be generated as seen at a center of the FIG. 2B. An image formed with aggregates of dots of such a state have a concentration distribution as shown in FIG. 2C in an ejection port arrangement direction, resulting in the unevenness of density that can be perceived by human eyes.
Then, the following method is known as a measure for such an unevenness of density. This method will be described with reference to FIG. 3 and FIG. 4.
According to the method, as shown in FIG. 3, the head 201 is scanned three times to accomplish a printed area shown in FIG. 1 or FIG. 2, a half of which, the area of four pixels, is accomplished by two scans (hereinafter referred to as "two passes"). In this case, eight ink ejection ports 202 of the head 201 are divided into two groups of upper four ejection ports and lower four ejection ports, and a number of dots formed of the inks ejected from a single ejection port in a single scan is thinned to about a half in the main scanning direction arrangement of dots. The remaining half dots are complementarily formed in the second scan to accomplish printing of the area of four pixels. Hereinafter referred to as the multi-scanning method.
Since, with this printing method, affection on the printing area inherent to each ejection port is reduced to a half even if the same head as shown in FIG. 2 is used, the printed image as shown in FIG. 3B can be obtained, in which black stripes or white stripes as seen in FIG. 2B are not noted. Therefore, as shown in FIG. 3C, the unevenness of density is also relatively well reduced as compared with the case shown in FIG. 2.
When performing such printing, with respect to the first and second scans, image data is divided according to a predetermined arrangement so that the image data for respective first and second scans complement each other. In most cases, the arrangement (thinning pattern) of the image data is formed just like in a checkered pattern at every pixel in horizontal and vertical directions, as shown in FIG. 4. In a unit printing area (in this case, the area of four pixels), a checkered pattern is printed in the first scan (FIG. 4A or 4C), and an inverse checkered pattern is printed in the second scan (FIG. 4B). Thus printing for the unit area is completed.
Further, normally, a constant amount of transporting a printing medium is set for respective transportations performed between respective two scans. In examples shown in FIG. 3 and FIG. 4, the printing medium is transported at an amount of 4 ejection port pitches.
As an example of high-speed printing technology, a construction with increased number of ejection ports of the printing head may be considered. In the case of serial printing in which printing is performed in one scan using all of the ejection ports of the printing head, although there is not a strict proportional relationship because of time required for supplying and discharging the printing medium or the like, a printing speed increases with increase of a number of ejection ports used for printing. For example, when using the printing head having 64 ejection ports at an arrangement density of 360 dpi, printing on an A4-size printing medium can be achieved in about 60 printing scans, whereas with a printing head having 256 ejection ports at the same arrangement density, printing is achieved in about 15 printing scans. The printing speed is increased as a whole to nearly about 4 times the former speed.
In this case, however, while the former head has an ejection port arrangement length of about 4.5 mm (=25.4 mm/360 dpi.times.63 ejection port pitches), the latter head has a port arrangement length of about 18 mm (=25.4 mm/360 dpi.times.255 ejection port pitches), which is about four times the former length. In principle, an accuracy of application position of printing dot is not changed depending upon the ejection port arrangement length. However, in an actual apparatus, an accuracy in fabrication process must be considered for mass production. For example, as shown in FIG. 5, a deviation may be generated due to a tolerance in a mounting angle of the printing head. For example, when the printing head mounting angle is slanted by 0.3.degree. with respect to a column direction on the image as a reference, the printing head having 64 ejection ports has a maximum deviation of about 23 .mu.m (=1.5 mm.times.sin 0.30.degree.), whereas the printing head having 256 ejection ports has a maximum deviation of about 94 .mu.m (=18 mm.times.sin 0.30.degree.). In the image having pixel density of 360 dpi, a pixel pitch is about 70 .mu.m. In this case, the above deviation is not a problem in a printing head of 64 ports. However, in a printing head of 256 ports, the deviation is greater than 1 pixel pitch, which is a fatal problem.
Further, when the printing head itself is curved as shown in FIG. 6, or similarly when the ejection direction of the ink is deflected, deviation of application position becomes conspicuous depending on the ejection port arrangement length which may be a major defect in the image.
In the case where deviation of application position due to the ejection port arrangement length is conspicuous, when printing by the above-described multi-scan method in order to eliminate the unevenness of density, a raster of the image is formed by different ejection ports. With respect to these ejection ports, the above-described deviation of application position in FIGS. 5 and 6 is a regular one shown in accordance with positions of the ejection ports used for printing the raster of the of image. Therefore, influence of such a deviation is less reduced by the multi-scan method and the deviation of application position is not eliminated, and a texture (pattern) is generated due to the deviation.
That is, in the prior art multi-scan method, a sufficient effect can be obtained for preventing unevenness of density due to the deviations in ejection characteristics of ejection ports. However, when an increased number of ejection ports are used for high-speed printing, the texture may be generated due to the deviation of application position caused by the ejection port arrangement length in the printing head.